1. Field of the Invention
The present invention relates to a hybrid type integrated optical device, which has a semiconductor laser mounted on a planar waveguide platform by flip-chip bonding, and more particularly to a hybrid type integrated optical device, designed to allow effective vertical alignment of an optical axis of a semiconductor laser and to reproducibly reduce an air gap between a planar waveguide platform and the semiconductor laser so as to provide high optical coupling efficiency between the planar waveguide platform and the semiconductor laser.
2. Description of the Related Art
Hybrid type integrated optical devices are generally manufactured by coupling a semiconductor laser and a planar waveguide platform by flip-chip bonding, and are used for various optical applications. Common examples include 1.31 μm/1.55 μm bi-directional optical transceiver modules, 1.31 μm/1.49 μm/1.55 μm triplexer optical transceiver modules, high velocity channel selectors using semiconductor optical amplifiers, multiple-channel optical transceiver module/optical monitor devices, which have a waveguide arrangement diffraction grating and a semiconductor laser chip or a semiconductor optical detector chip integrated thereon, and the like.
FIG. 1 is a cross-sectional view of a conventional hybrid type integrated optical element. Referring to FIG. 1, the conventional hybrid type integrated optical element comprises a semiconductor laser 11 mounted on a predetermined region A of a planar waveguide platform 12 by flip-chip bonding.
The planar waveguide platform 12 has a substrate 121, a lower clad layer 122, a core layer 123, and an upper clad layer 124 sequentially stacked in this order on the substrate 121, and has a region A for mounting the semiconductor laser 11 thereon using flip-chip bonding. The region A to which the semiconductor laser 11 is flip-chip bonded can be formed by selectively removing a predetermined portion of the lower clad layer 122, the core layer 123, and the upper clad layer on the predetermined region A after laminating the lower clad layer 122, the core layer 123, and the upper clad layer on the substrate 121. Then, a metallic pattern, an alignment pattern, and the like are formed on the flip-chip bonded region A by a semiconductor photolithography process.
The semiconductor laser 11 is welded to an upper surface of the flip-chip bonded region A on the planar waveguide platform 12 by use of a welding metal 13. In general, the welding metal 13 includes under bump metal (UMB), and a solder (Au/Sn). Upon flip-chip bonding, the welding metal 13 is heated to a temperature of about 280° C. or more, and fused, thereby allowing the semiconductor laser 11 to be welded to the substrate 121 of the planar waveguide platform 12.
Light generated from an active region 112 of the semiconductor laser 11 is optically coupled to a side surface 128 of the planar waveguide platform 12 through a light emission surface 118 of the semiconductor laser 11. The light emission surface 118 of the semiconductor laser 11 and the other side surface opposite to the light emission surface 118 have an antireflection film and a high-reflection film coated thereon, respectively, according to performance and objects of devices to be manufactured.
As shown in FIG. 1, when manufacturing the hybrid type integrated optical device, which has the semiconductor laser 11 bonded to the predetermined region A of the planar waveguide platform 12 in a passive alignment by flip-chip bonding, it is necessary to provide accurate alignment of an optical axis in the vertical and horizontal directions for ensuring effective optical coupling, and to reduce an air gap 14 between the light emission surface 118 of the semiconductor laser 11 and the side surface 128 of the planar waveguide platform 12 facing each other.
Conventionally, a silicon or silica terrace has been provided to a substrate of the optical device for alignment of the optical axis in the vertical direction, so that the semiconductor laser is flip-chip bonded to the terrace so as to be flush with the planar waveguide in the vertical direction, and an alignment mark has been used for alignment of the optical axis in the horizontal direction.
When welding the semiconductor laser 11 to the substrate by the flip-chip bonding method, an alignment error, which can be created by the conventional optical alignment method, is within ±2 μm in the vertical/horizontal directions. In order to prevent reduction in optical coupling efficiency due to such an alignment error, conventionally, an optical mode size converter (which is also referred to as a “spot size converter”) 114 is provided within the semiconductor laser 11, and converts a spot size of light output from the active region 112 to a larger spot size of light. The optical mode size converter 114 acts to convert an optical mode (that is, spot size) of light generated from the active region 112 of the semiconductor laser 11, and to transfer the converted light to the light emission surface 118.
Typically, the optical mode size converter 114 is provided in the waveguide by reducing the size of the waveguide in the vertical and/or horizontal directions. The performance of the optical mode size converter 114 can be evaluated with a far-field angle. At this time, for a wide far-field angle, the optical coupling efficiency is varied depending on the size of the air gap 14 between the light emission surface 118 of the semiconductor laser 11 and the side surface 128 of the planar waveguide platform 12 facing each other. Thus, in order to achieve reproducible optical coupling efficiency, the air gap 14 must be accurately controlled in size upon flip-chip bonding.
However, in a typical process of manufacturing the semiconductor laser 11, since a single chip is provided by cleaving a plurality of semiconductor chips formed on a wafer, an error of about ±30 μm is created to the length of the semiconductor laser 11 with reference to a target value. Due to such an error in the process of manufacturing the semiconductor laser 11, the air gap 14 can have a size varied in the range of about ±30 μm, thereby causing a problem of variation up to 50%, in optical coupling efficiency between the semiconductor laser 11 and the planar waveguide 12.
In order to solve the problem, there has been investigation into integration of an optical mode size converter 114, which reduces the far-field angle of light transferred through the light emission surface 118 of the semiconductor laser 11 to the maximum extent, into a semiconductor laser chip. With regard to this, it is necessary to accurately form tapers of the waveguide of the optical mode size converter 114 in both vertical and horizontal directions in order to reduce the far field angle. Additionally, since internal reflection can occur at a butt-joint region 116 where the waveguide of the active region 112 of the semiconductor laser 11 meets the waveguide within the optical mode size converter 114, it is necessary to provide accurate control over deposition and patterning processes for respective regions. As such, accuracy required for the process of manufacturing the optical mode size converter causes a problem of raising the price of the semiconductor together with the price of the hybrid type integrated optical device manufactured by use of the semiconductor laser.